.TH std::atomic_compare_exchange_weak,std::atomic_compare_exchange_strong, 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::atomic_compare_exchange_weak,std::atomic_compare_exchange_strong, \- std::atomic_compare_exchange_weak,std::atomic_compare_exchange_strong,

.SH Synopsis

   Defined in header <atomic>
   template< class T >

   bool atomic_compare_exchange_weak
       ( std::atomic<T>* obj, typename std::atomic<T>::value_type*    \fB(1)\fP \fI(since C++11)\fP
   expected,

         typename std::atomic<T>::value_type desired ) noexcept;
   template< class T >

   bool atomic_compare_exchange_weak
       ( volatile std::atomic<T>* obj,                                \fB(2)\fP \fI(since C++11)\fP
         typename std::atomic<T>::value_type* expected,

         typename std::atomic<T>::value_type desired ) noexcept;
   template< class T >

   bool atomic_compare_exchange_strong
       ( std::atomic<T>* obj, typename std::atomic<T>::value_type*    \fB(3)\fP \fI(since C++11)\fP
   expected,

         typename std::atomic<T>::value_type desired ) noexcept;
   template< class T >

   bool atomic_compare_exchange_strong
       ( volatile std::atomic<T>* obj,                                \fB(4)\fP \fI(since C++11)\fP
         typename std::atomic<T>::value_type* expected,

         typename std::atomic<T>::value_type desired ) noexcept;
   template< class T >

   bool atomic_compare_exchange_weak_explicit
       ( std::atomic<T>* obj, typename std::atomic<T>::value_type*
   expected,                                                          \fB(5)\fP \fI(since C++11)\fP
         typename std::atomic<T>::value_type desired,

         std::memory_order success, std::memory_order failure )
   noexcept;
   template< class T >

   bool atomic_compare_exchange_weak_explicit
       ( volatile std::atomic<T>* obj,
         typename std::atomic<T>::value_type* expected,               \fB(6)\fP \fI(since C++11)\fP
         typename std::atomic<T>::value_type desired,

         std::memory_order success, std::memory_order failure )
   noexcept;
   template< class T >

   bool atomic_compare_exchange_strong_explicit
       ( std::atomic<T>* obj, typename std::atomic<T>::value_type*
   expected,                                                          \fB(7)\fP \fI(since C++11)\fP
         typename std::atomic<T>::value_type desired,

         std::memory_order success, std::memory_order failure )
   noexcept;
   template< class T >

   bool atomic_compare_exchange_strong_explicit
       ( volatile std::atomic<T>* obj,
         typename std::atomic<T>::value_type* expected,               \fB(8)\fP \fI(since C++11)\fP
         typename std::atomic<T>::value_type desired,

         std::memory_order success, std::memory_order failure )
   noexcept;

   Atomically compares the
   object representation
   \fI(until C++20)\fP
   value representation
   \fI(since C++20)\fP of the object pointed to by obj with that of the object pointed to by
   expected, and if those are bitwise-equal, replaces the former with desired (performs
   read-modify-write operation). Otherwise, loads the actual value pointed to by obj
   into *expected (performs load operation).

           Overloads                      Memory model for
                       read‑modify‑write operation        load operation
          (1-4)       std::memory_order_seq_cst      std::memory_order_seq_cst
          (5-8)       success                       failure

   These functions are defined in terms of member functions of std::atomic:

   1,2) obj->compare_exchange_weak(*expected, desired)
   3,4) obj->compare_exchange_strong(*expected, desired)
   5,6) obj->compare_exchange_weak(*expected, desired, success, failure)
   7,8) obj->compare_exchange_strong(*expected, desired, success, failure)

   If failure
   is stronger than success or
   \fI(until C++17)\fP is one of std::memory_order_release and std::memory_order_acq_rel, the
   behavior is undefined.

.SH Parameters

   obj      - pointer to the atomic object to test and modify
   expected - pointer to the value expected to be found in the atomic object
   desired  - the value to store in the atomic object if it is as expected
   success  - the memory synchronization ordering for the read-modify-write operation
              if the comparison succeeds
   failure  - the memory synchronization ordering for the load operation if the
              comparison fails

.SH Return value

   The result of the comparison: true if *obj was equal to *expected, false otherwise.

.SH Notes

   std::atomic_compare_exchange_weak and std::atomic_compare_exchange_weak_explicit
   (the weak versions) are allowed to fail spuriously, that is, act as if *obj !=
   *expected even if they are equal. When a compare-and-exchange is in a loop, they
   will yield better performance on some platforms.

   When a weak compare-and-exchange would require a loop and a strong one would not,
   the strong one is preferable unless the object representation of T may include
   padding bits,
   \fI(until C++20)\fP trap bits, or offers multiple object representations for the same
   value (e.g. floating-point NaN). In those cases, weak compare-and-exchange typically
   works because it quickly converges on some stable object representation.

   For a union with bits that participate in the value representations of some members
   but not the others, compare-and-exchange might always fail because such padding bits
   have indeterminate values when they do not participate in the value representation
   of the active member.

   Padding bits that never participate in an object's value               \fI(since C++20)\fP
   representation are ignored.

.SH Example

   Compare and exchange operations are often used as basic building blocks of lockfree
   data structures.


// Run this code

 #include <atomic>

 template<class T>
 struct node
 {
     T data;
     node* next;
     node(const T& data) : data(data), next(nullptr) {}
 };

 template<class T>
 class stack
 {
     std::atomic<node<T>*> head;
 public:
     void push(const T& data)
     {
         node<T>* new_node = new node<T>(data);

         // put the current value of head into new_node->next
         new_node->next = head.load(std::memory_order_relaxed);

         // now make new_node the new head, but if the head
         // is no longer what's stored in new_node->next
         // (some other thread must have inserted a node just now)
         // then put that new head into new_node->next and try again
         while (!std::atomic_compare_exchange_weak_explicit(
                    &head, &new_node->next, new_node,
                    std::memory_order_release, std::memory_order_relaxed))
             ; // the body of the loop is empty
 // note: the above loop is not thread-safe in at least
 // GCC prior to 4.8.3 (bug 60272), clang prior to 2014-05-05 (bug 18899)
 // MSVC prior to 2014-03-17 (bug 819819). See member function version for workaround
     }
 };

 int main()
 {
     stack<int> s;
     s.push(1);
     s.push(2);
     s.push(3);
 }

   Defect reports

   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.

     DR    Applied to         Behavior as published         Correct behavior
   P0558R1 C++11      exact type match was required because T is only deduced
                      T was deduced from multiple arguments from obj

.SH See also

                            atomically compares the value of the atomic object with
   compare_exchange_weak    non-atomic argument and performs atomic exchange if equal
   compare_exchange_strong  or atomic load if not
                            \fI(public member function of std::atomic<T>)\fP
   atomic_exchange          atomically replaces the value of the atomic object with
   atomic_exchange_explicit non-atomic argument and returns the old value of the atomic
   \fI(C++11)\fP                  \fI(function template)\fP
   \fI(C++11)\fP

   std::atomic_compare_exchange_weak(std::shared_ptr)   specializes atomic operations
   std::atomic_compare_exchange_strong(std::shared_ptr) for std::shared_ptr
   (deprecated in C++20)                                \fI(function template)\fP
   (removed in C++26)
   C documentation for
   atomic_compare_exchange,
   atomic_compare_exchange_explicit
